The present invention relates generally to the removal of hydrogen sulfide from gas streams by contacting such gas streams with a regenerable, aqueous washing solution. More particularly, the present invention relates to the reconditioning of such a washing solution after it has become contaminated with sulfur-feeding bacteria and/or to the maintenance of such a washing solution in a substantially bacteria-free condition.
With the increasing concern over atmospheric pollution and the concomitant increasingly strict enforcement of tight air pollution standards, ever greater responsibility is placed on industry to produce pollution-free products in a non-polluting manner. An area of particular environmental concern has been, and is, the discharge or release of sulfur and its compounds, especially hydrogen sulfide, into the atmosphere as a result of various industrial processes. These processes include, for example, petroleum refining, the sweetening of sour natural gas, destructive distillation of coal and oil shale, gasification or liquefication of coal and production and use of hydrogen sulfide-containing geothermal steam and liquid for generating electricity or for other uses.
Several processes have been developed, and are in relatively common use, for removing hydrogen sulfide from gas streams such as those generated and/or encountered in the industrial processes listed above. Such hydrogen sulfide removal processes as the Stretford and the Takahax processes, employ an aqueous, alkaline washing solution to absorb hydrogen sulfide from a gas stream, for example, a tail gas from a Claus apparatus. Vanadates in the washing solution convert the hydrogen sulfide to elemental sulfur, which is then removed in particulate form from the washing solution. The sulfur removal may be accomplished in an autoclave in which small sulfur particles are agglomerated for removal purposes. Washing solutions employed in these hydrogen sulfide removal processes are typically regenerated for reuse by injecting air into and through the solutions, the oxygen contained in the air reoxidizing the vanadate or other metal compound used.
By way of background, the washing solution employed in the Stretford process contains a water-soluble salt of 9,10 anthraquinone disulfonic acid (ADA) and a water-soluble vanadate (or other multivalent metal) compound. The Takahax process, in contrast, uses a washing solution containing a water-soluble naphthaquinone sulfonate (NQS), alone or in combination with a multivalent metal compound. The hydrogen sulfide removal process disclosed in Fenton et al. U.S. Pat. No. 4,283,379 employs a washing solution containing solubilized vanadium, one or more water-soluable non-quinone aromatic compounds, thiocyanate ions and a water-soluble carboxylate complexing agent.
It is known in the petroleum industry that certain types of bacteria which live on sulfur can cause serious problems in those petroleum operations in which sulfur and/or sulfur compounds are present. Kreuz et al., U.S. Pat. No. 3,329,610 for example, discusses the bacterial problems encountered in secondary oil recovery operations employing water flooding or injection. Sulfur-feeding bacteria in these secondary oil recovery operations have been found to cause severe corrosion of metal equipment and, when present in sufficiently large numbers, to cause plugging of pores in the associated oil bearing earth formations. The two classes of bacteria believed responsible for these problems are identified in the Kreuz et al. patent as Disulforibrio desulfuricans (sulfate reducers), an anerobe, and Pseudomonas, an aerobe.
Recently, sulfur-feeding bacteria have been discovered in certain hydrogen sulfide gas removal systems, namely Stretford process systems. Although some hydrogen sulfide washing solutions, such as those disclosed in the above-cited patent to Fenton et al., are considered hostile to living organisms because they contain thiocyanate compounds, other hydrogen sulfide washing solutions, such as those used in the Stretford process, appear to support the growth of at least some types of sulfur-feeding bacteria. The Stretford process washing solutions, because of their hot, aqueous nature, appear to promote the breeding of sulfur-feeding bacteria to an extent that severe problems with hydrogen sulfide removal may result.
In specific respect to the observed sulfur bacteria problems in Stretford process washing solutions, bacteria entering the elemental sulfur-removing autoclave are apparently killed by the increased temperature encountered in the autoclave. After being killed, however, the dead bacteria have been found to remain adhered to the sulfur particles on which they had been feeding and thereby form a coating which prevents the sulfur particles from agglomerating in the normal manner required for sulfur removal. As a consequence, the sulfur particles are recirculated with the washing solution from the autoclave back into the rest of the system and interfere with the hydrogen sulfide gas removal process.
The living bacteria in the system, particularly in the solution regeneration (vanadate reoxidation) vessel, have been found to produce large amounts of slime and to cause excessive solution frothing, both effects inhibiting the solution regeneration process. Since oxygen and elemental sulfur are present in the regeneration vessel, the vessel is considered a good breeding ground for the sulfur-feeding bacteria, the bacteria being bred in the vessel at a faster rate than bacteria are killed in the autoclave. Since the washing solution is also recirculated from the regeneration vessel, sulfur-feeding bacteria are continually circulated through the entire system from the vessel.
Due to the effects of the sulfur-feeding bacteria, the hydrogen sulfide removal capacity of the washing solution rapidly decreases as the bacterial contamination increases. As a result, the industrial process with which the hydrogen sulfide removal process is associated must usually be curtailed with production being lost and/or production costs being increased.
Until recently, degradation of hydrogen sulfide washing solutions has not been attributed to sulfur-feeding bacteria. Most attempts to alleviate problems of washing solution degradation are believed to have involved replacing only a portion of the degraded washing solution with fresh washing solution. However, in the case of bacterial contamination of the washing solution, replacement of only a portion of the washing solution would result in bacterial contamination of the newly added solution, and any beneficial effects of adding the fresh solution would be only temporary.
Since hydrogen sulfide removal systems, such as Stretford process systems, typically employ between about 100,000 and 500,000 gallons of washing solution, total washing solution replacement is relatively costly. Substantial additional costs may be associated with disposal of the replaced washing solution. As a result, replacement of the entire washing solution is not economically attractive, even if it were possible to clean the system sufficiently to remove all traces of bacteria which could cause infestation of the replacement washing solution. Also, considerable down time would be required for the cleaning operation.
In respect to known sulfur feeding bacterial problems associated with water flooding or injection for secondary oil recovery, the above-identified patent to Kreuz et al., as well as U.S. Pat. Nos. 2,987,475 and 3,049,492 to Legator and DeGroote et al., respectively, disclose use of specific bactericides with which the water may be treated as a preventative measure. Use of peracetic acid, olefinically unsaturated lower alkyl aldehydes and certain oxirane ring containing compounds as bactericides are disclosed in the patents mentioned above. However, none of these disclosed bactericides is considered adaptable for use in hydrogen sulfide washing solutions, because none of these bactericides is chemically stable at the conditions existant within typical hydrogen sulfide washing solutions. In such washing solutions each of these bactericides rapidly reacts with chemical constituents present in the washing solution, thereby decomposing into less bactericidally active materials. Peracetic acid, for example, reacts with hydrogen sulfide and the reduced form of the multivalent metal ion (V.sup.+4 in Stretford solutions) to form relatively bactericidally inactive acetic acid. Olefinically unsaturated lower alkyl aldehydes react with gaseous oxygen to form bactericidally active peracids but, as is the case with peracetic acid, these peracids react with hydrogen sulfide and the reduced form of the multivalent metal to form relatively bactericidally inactive organic acids. Oxirane rings react with water to form relatively bactericidally inactive glycols.
A significant aspect of the problem of bacterial contamination or infestation of hydrogen sulfide removal systems, such as Stretford process systems, is that the contamination problem is not universal but seems to occur only in some hydrogen sulfide removal systems. Although the bacterial contaminations appear most often in systems located in hot, moist climates, as may be found, for example, around the Gulf of Mexico in the United States, not all similar systems in this type climate become contaminated. Furthermore, the bacterial infestations appear to be cyclic in nature and may be dependent upon as yet unknown cyclic characteristics of the bacteria. As a result, even after the bacterial nature of the hydrogen sulfide removal process has been identified, universally applied preventive techniques do not appear economically feasible.
An important need therefore exists for an effective economical method for reconditioning hydrogen sulfide removal systems which have become infested or contaminated by sulfur-feeding bacteria to an extent that effectiveness of the hydrogen sulfide removal system is substantially impaired.
Accordingly, it is a principal object of the present invention to provide a method for reconditioning hydrogen sulfide removal systems and washing solutions which have become badly infested with sulfur-feeding bacteria.
Still another object of the invention is to prevent subsequent bacterial reinfestation of the hydrogen sulfide removal system and washing solution by the sulfur-feeding bacteria after the system and solution have been reconditioned.
Further objects, advantages and features of the invention will become apparent to those skilled in the art from the following description when taken in conjunction with the accompanying drawing.